One of the problems with casting materials that expand during solidification is that the solidified materials often develop cracks or fractures that reduce the effectiveness and appeal of the cast product as well as the integrity of the product. This is particularly true in the sporting industry where bismuth and bismuth alloy metals are being touted to replace lead and lead alloy articles such as fishing sinkers. In order to remove such a cast article from a mold, the molds have been made in multiple parts that allow the mold parts to be removed in sections thereby preventing the cast article from being retained in the mold cavity due to an interference fit between the mold surfaces and the surfaces of the solidified cast article.
In contrast to the above method of separating the mold parts after a cast article has been cast, the present invention relates to formation of cast article by use of a mold cavity that expands during the solidification phase to maintain or limit the increase in an internal pressure in the mold cavity and hence in the cast article. The pressure within the cavity is maintained at a pressure less than what can be consider a “fracture pressure” during the solidification phase of the cast article but equal or greater than the injection molding pressure. By fracture pressure it is meant that if the cast article solidified under such pressure that the cast article would contain cracks or voids that render the cast article undesirable for the intended use.
During the casting of parts with materials such as bismuth and bismuth alloys in a fixed volume mold it has been found that there are voids or cracks in the finished part. Normally, a void or crack in a finished part is a result of having insufficient material in the mold cavity as the metal solidifies. In the present invention, it appears that the voids or cracks are not formed as a result of having an insufficient amount of material in the mold cavity but as a result of the increase of the pressure during the solidification phase. While the exact mechanism of why the increase in solidification pressure can result in cracks and voids in the finished product is not fully understood, the present invention provides a method and apparatus that inhibits or eliminates the voids or cracks in the finished product by reducing mold pressure on the article. In the preferred embodiment a mold having a mold cavity that expands during the solidification phase to limit an increase in internal pressure in the cast article during the solidification phase is used to form a cast article.